Where can i find the maximum amount an expert can charge on a civil legal aid case?
There is no maximum. The rate depends on a number of factors - complexity, status of expert, time spent, market forces, etc.
The amount is negotiable with most experts.
You also need to remember that a good expert, doing a proper job, can be worth their weight in gold. If it makes the difference between winning and losing, most lawyers recommend getting the best expert available, who will accept a legal aid briefing. If the case is won, then there is every chance that the other side will have to pay the costs of the expert in any event, provided that they are reasonable.